diff --git a/src/u_sxsyn.pas b/src/u_sxsyn.pas
index c13f30aa..200c3783 100644
--- a/src/u_sxsyn.pas
+++ b/src/u_sxsyn.pas
@@ -525,6 +525,8 @@ begin
 end;
 
 procedure TSynSxSyn.lexHexLiteral();
+var
+  firstChar: Boolean = false;
 begin
   fTokStop += 2;
   fTokKind:=TTokenKind.tkNumbr;
@@ -533,7 +535,10 @@ begin
     case fLineBuf[fTokStop] of
       '0'..'9', 'a'..'f', 'A'..'F', '_':
       begin
+        if not firstChar and (fLineBuf[fTokStop] = '_') then
+          fTokKind:=TTokenKind.tkError;
         fTokStop += 1;
+        firstChar := true;
         continue;
       end
       else while (fTokStop <= fLineBuf.length) and
@@ -548,6 +553,8 @@ begin
 end;
 
 procedure TSynSxSyn.lexBinLiteral();
+var
+  firstChar: Boolean = false;
 begin
   fTokStop += 2;
   fTokKind:=TTokenKind.tkNumbr;
@@ -556,7 +563,10 @@ begin
     case fLineBuf[fTokStop] of
       '0', '1', '_':
       begin
+        if not firstChar and (fLineBuf[fTokStop] = '_') then
+          fTokKind:=TTokenKind.tkError;
         fTokStop += 1;
+        firstChar := true;
         continue;
       end;
       else while (fTokStop <= fLineBuf.length) and